Bitcoin

There are minor differences between the variant of the hashcash mining
function used for X-Hashcash mail stamps and the one in bitcoin: a) hashash
difficulty can only double or halve, bitcoin uses more fine grained
difficulty adjustment; b) bitcoin is updated to use the SHA256 hash vs SHA1
hash in hashcash; c) bitcoin runs the hash twice for added security margin.

Hashcash difficulty is
static and eroded by Moore's law currently 20 bits.

Inflation vs deflation: the only current mechanism to upgrade hashcash
default (20 bits) is via a sofware update though it can be overridden on the
command line. Bitcoin on the otherhand has inflation control, with a
dynamically adjusted difficulty aiming at a fixed rate of production.
Inflation control is a major innovation of bitcoin over hashcash, and RPOW.

Note hashcash is just the mining function used by bitcoin.
Bitcoin, which is the work of the psuedonymous Satoshi Nakamoto, is
an extremely clever innovation and invention comprising multiple features
and cutting edge new concepts, not fully realised in any previous electronic
cash scheme. Bitcoin represents a leap forward in electronic cash
technology demonstrating for the first time that a respendable, distributed,
virtual scarcity based system could be built. There were earlier
ideas that are similar however as far as I could gather Nakamoto
was not aware of B-money, and presumably not bit-gold either has he does not
reference that in his paper. The four main features of the bitcoin network
are a public transaction ledger (in fact a transaction log because it
exhibits cryptographically enforced append only properties), a p2p network
for p2p transactions and distributed management of the security of the
transaction log, a novel inflation controlled whole network mining
difficulty allowing the creation of virtual scarce bitcoins, and finally
smart contracts. Even without smart contracts thats a first. Smart
contracts are icing on top, and also a first, with publicly auditable
self-execuing smart contract.

Because of the byzantine threat models in maintaining a distributed
transaction database, votes are placed on its correctness in proportion to
the computational power of the clients. There is a reward for the
computational power which is to create virtual, scarce, bitcoins at a fixed
controlled rate, with difficulty of the work dynamically adjusted to keep
the rate of production approximately fixed as the computational power of the
network grows or shinks.